Predicted hardness of Austempered Vermicular Graphite Iron
DOI:
https://doi.org/10.24425/afe.2025.155350Abstract
The aim of this study was to determine the hardness of vermicular cast iron subjected to austempering, depending on the parameters of the heat treatment process. The heat treatment was conducted based on orthogonal experimental design, with a total of 27 experiments performed. The samples underwent austenitization at temperatures of 890°C, 925°C, and 960°C, followed by austempering at 290°C, 340°C, and 390°C. The austenitization and austempering times were set to 90 min, 120 min, and 150 min. To analyse the influence of these parameters, a full polynomial regression model was developed. The proposed model, which describes the hardness of the cast iron after heat treatment, showed a predicted coefficient of determination (R²) of approximately 78%. For optimization purposes, the Response Surface Methodology (RSM) was employed. The results of the ANOVA analysis indicated that the austempering temperature (Tpi), the square of the austenitization time (τγ²), the interaction between austenitization temperature and time (Tγ τγ), as well as the interaction between austenitization and austempering temperatures (Tγ Tpi) had the most significant impact on the examined parameter. Following variance analysis, the model was refined once more to eliminate insignificant predictors. The simplified model improved the predicted coefficient of determination to 93%. The optimal conditions for the analyzed parameters, assuming a maximum hardness of approximately 440 HB, were obtained under the following heat treatment conditions: Tγ = 930°C, Tpi = 290°C, τγ = 150 min, and τpi = 150 min.References
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